---
title: Apply Interface Segregation Principle
impact: HIGH
impactDescription: Reduces coupling and improves testability by 30-50%
tags: dependency-injection, interfaces, solid, isp
---

## Apply Interface Segregation Principle

Clients should not be forced to depend on interfaces they don't use. In NestJS, this means keeping interfaces small and focused on specific capabilities rather than creating "fat" interfaces that bundle unrelated methods. When a service only needs to send emails, it shouldn't depend on an interface that also includes SMS, push notifications, and logging. Split large interfaces into role-based ones.

**Incorrect (fat interface forcing unused dependencies):**

```typescript
// Fat interface - forces all consumers to depend on everything
interface NotificationService {
  sendEmail(to: string, subject: string, body: string): Promise<void>;
  sendSms(phone: string, message: string): Promise<void>;
  sendPush(userId: string, notification: PushPayload): Promise<void>;
  sendSlack(channel: string, message: string): Promise<void>;
  logNotification(type: string, payload: any): Promise<void>;
  getDeliveryStatus(id: string): Promise<DeliveryStatus>;
  retryFailed(id: string): Promise<void>;
  scheduleNotification(dto: ScheduleDto): Promise<string>;
}

// Consumer only needs email, but must mock everything for tests
@Injectable()
export class OrdersService {
  constructor(
    private notifications: NotificationService, // Depends on 8 methods, uses 1
  ) {}

  async confirmOrder(order: Order): Promise<void> {
    await this.notifications.sendEmail(
      order.customer.email,
      'Order Confirmed',
      `Your order ${order.id} has been confirmed.`,
    );
  }
}

// Testing is painful - must mock unused methods
const mockNotificationService = {
  sendEmail: jest.fn(),
  sendSms: jest.fn(),           // Never used, but required
  sendPush: jest.fn(),          // Never used, but required
  sendSlack: jest.fn(),         // Never used, but required
  logNotification: jest.fn(),   // Never used, but required
  getDeliveryStatus: jest.fn(), // Never used, but required
  retryFailed: jest.fn(),       // Never used, but required
  scheduleNotification: jest.fn(), // Never used, but required
};
```

**Correct (segregated interfaces by capability):**

```typescript
// Segregated interfaces - each focused on one capability
interface EmailSender {
  sendEmail(to: string, subject: string, body: string): Promise<void>;
}

interface SmsSender {
  sendSms(phone: string, message: string): Promise<void>;
}

interface PushSender {
  sendPush(userId: string, notification: PushPayload): Promise<void>;
}

interface NotificationLogger {
  logNotification(type: string, payload: any): Promise<void>;
}

interface NotificationScheduler {
  scheduleNotification(dto: ScheduleDto): Promise<string>;
}

// Implementation can implement multiple interfaces
@Injectable()
export class NotificationService implements EmailSender, SmsSender, PushSender {
  async sendEmail(to: string, subject: string, body: string): Promise<void> {
    // Email implementation
  }

  async sendSms(phone: string, message: string): Promise<void> {
    // SMS implementation
  }

  async sendPush(userId: string, notification: PushPayload): Promise<void> {
    // Push implementation
  }
}

// Or separate implementations
@Injectable()
export class SendGridEmailService implements EmailSender {
  async sendEmail(to: string, subject: string, body: string): Promise<void> {
    // SendGrid-specific implementation
  }
}

// Consumer depends only on what it needs
@Injectable()
export class OrdersService {
  constructor(
    @Inject(EMAIL_SENDER) private emailSender: EmailSender, // Minimal dependency
  ) {}

  async confirmOrder(order: Order): Promise<void> {
    await this.emailSender.sendEmail(
      order.customer.email,
      'Order Confirmed',
      `Your order ${order.id} has been confirmed.`,
    );
  }
}

// Testing is simple - only mock what's used
const mockEmailSender: EmailSender = {
  sendEmail: jest.fn(),
};

// Module registration with tokens
export const EMAIL_SENDER = Symbol('EMAIL_SENDER');
export const SMS_SENDER = Symbol('SMS_SENDER');

@Module({
  providers: [
    { provide: EMAIL_SENDER, useClass: SendGridEmailService },
    { provide: SMS_SENDER, useClass: TwilioSmsService },
  ],
  exports: [EMAIL_SENDER, SMS_SENDER],
})
export class NotificationModule {}
```

**Combining interfaces when needed:**

```typescript
// Sometimes a consumer legitimately needs multiple capabilities
interface EmailAndSmsSender extends EmailSender, SmsSender {}

// Or use intersection types
type MultiChannelSender = EmailSender & SmsSender & PushSender;

// Consumer that genuinely needs multiple channels
@Injectable()
export class AlertService {
  constructor(
    @Inject(MULTI_CHANNEL_SENDER)
    private sender: EmailSender & SmsSender,
  ) {}

  async sendCriticalAlert(user: User, message: string): Promise<void> {
    await Promise.all([
      this.sender.sendEmail(user.email, 'Critical Alert', message),
      this.sender.sendSms(user.phone, message),
    ]);
  }
}
```

Reference: [Interface Segregation Principle](https://en.wikipedia.org/wiki/Interface_segregation_principle)